Anesthetic potency and cardiopulmonary effects of sevoflurane in goats: comparison with isoflurane and halothane.

The anesthetic potency and cardiopulmonary effects of sevoflurane were compared with those of isoflurane and halothane in goats. The (mean +/- SD) minimal alveolar concentration (MAC) was 0.96 +/- 0.12% for halothane, 1.29 +/- 0.11% for isoflurane, and 2.33 +/- 0.15% for sevoflurane. Cardiopulmonary effects of sevoflurane, halothane and isoflurane were examined at end-tidal concentrations equivalent to 1, 1.5 and 2 MAC during either spontaneous or controlled ventilation (SV or CV). During SV, there were no significant differences in respiration rate, tidal volume and minute ventilation between anesthetics. Dose-dependent decreases in both tidal volume and minute ventilation induced by halothane were greater than those by either sevoflurane or isoflurane. Hypercapnia and acidosis induced by sevoflurane were not significantly different from those by either isoflurane or halothane at 1 and 1.5 MAC, but were less than those by halothane at 2 MAC. There was no significant difference in heart rate between anesthetics during SV and CV. During SV, all anesthetics induced dose-dependent decreases in arterial pressure, rate pressure product, systemic vascular resistance, left ventricular minute work index and left ventricular stroke work index. Systemic vascular resistance with isoflurane at 2 MAC was lower than that with sevoflurane. During CV, sevoflurane induced dose-dependent circulatory depression (decreases in arterial pressure, cardiac index, rate pressure product, systemic vascular resistance, left ventricular minute work index and right ventricular minute work index), similar to isoflurane. Halothane did not significantly alter systemic vascular resistance from 1 to 2 MAC.     

Hepatic effects of halothane, isoflurane or sevoflurane anaesthesia in dogs

The effects of halothane, isoflurane and sevoflurane anaesthesia on hepatic function and hepatocellular damage were investigated in dogs, comparing the activity of hepatic enzymes and bilirubin concentration in serum. An experimental study was designed. Twenty-one clinically normal mongrel dogs were divided into three groups and accordingly anaesthetized with halothane (n = 7), isoflurane (n = 7) and sevoflurane (n = 7). The dogs were 1-4 years old, and weighed between 13.5 and 27 kg (18.4 +/- 3.9). Xylazine HCI (1-2 mg/kg) i.m. was used as pre-anaesthetic medication. Anaesthesia was induced with propofol 2 mg/kg i.v. The trachea was intubated and anaesthesia maintained with halothane, isoflurane or sevoflurane in oxygen at concentrations of 1.35, 2 and 3%, respectively. Intermittent positive pressure ventilation (tidal volume, 15 ml/kg; respiration rate, 12-14/min) was started immediately after intubation and the anaesthesia lasted for 60 min. Venous blood samples were collected before pre-medication, 24 and 48 h, and 7 and 14 days after anaesthesia. Serum level of aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP) and gamma-glutamyltransferase (GGT), lactate dehydrogenase (LDH GGT) activities and bilirubin concentration were measured. Serum AST, ALT and GGT activities increased after anaesthesia in all groups. In the halothane group, serum AST and ALT activities significantly increased all the time after anaesthesia compared with baseline activities. But in the isoflurane group AST and ALT activities increased only between 2 and 7 days, and in the sevoflurane group 7 days after anaesthesia. GGT activity was increased in the halothane group between 2 and 7 days, and in the isoflurane and sevoflurane groups 7 days after anaesthesia. All dogs recovered from anaesthesia without complications and none developed clinical signs of hepatic damage within 14 days. The results suggest that the use of halothane anaesthesia induces an elevation of serum activities of liver enzymes more frequently than isoflurane or sevoflurane from 2 to 14 days after anaesthesia in dogs. The effects of isoflurane or sevoflurane anaesthesia on the liver in dogs is safer than halothane anaesthesia in dogs.     

Analysis of the frequency spectrum of the equine electroencephalogram during halothane anaesthesia

The electroencephalogram (EEG) has been used in human clinical anaesthesia as an indicator of cortical activity and as an indicator of the depth of anaesthesia. It would be useful if it provided a reliable indication of the depth of anaesthesia of horses. In this study anaesthesia was induced with thiopentone and maintained with halothane in nine ponies. The end tidal halothane concentration (P_E-Hal) was monitored and 20 seconds of EEG were recorded at 0·8 per cent, 1·0 per cent and 1·2 per cent halothane, equivalent to the minimum alveolar concentration (mac), 1·25 mac and 1.5 mac. Each 20 second block of data was divided into one second segments and averaged to give one second of averaged EEG from which a frequency spectrum was obtained by using a fast Fourier transformation. The power of the waveform at low frequency (1 to 3 Hz) was compared with that at higher frequency (9 to 11 Hz). The median frequency and 95th percentile (spectral edge) were also calculated. The spectral edge frequency had the best correlation with P_E-Hal     

Comparisons of Sevoflurane, Isoflurane, and Halothane Anesthesia in Spontaneously Breathing Cats

The clinical effects of sevoflurane, isoflurane, and halothane anesthesia with or without nitrous oxide, were compared in healthy, premedicated cats breathing spontaneously during 90 minutes of anesthesia. The effect of nitrous oxide in accelerating the induction of and recovery from anesthesia was more evident for halothane than for sevoflurane or isoflurane. The cats recovered more rapidly from sevoflurane-oxygen than from either halothane- or isoflurane-oxygen. Heart rates did not significantly change during anesthesia with any of the anesthetics. Arterial blood pressures during sevoflurane-oxygen anesthesia were somewhat higher than those with either isoflurane- or halothane-oxygen. There were no significant differences in arterial blood pressures among sevoflurane, isoflurane, and halothane anesthesia when combined with nitrous oxide. The respiration rate during sevoflurane-oxygen was similar to that during halothane-oxygen. There were no significant differences in respiration rate among sevoflurane, isoflurane, and halothane anesthesia when combined with nitrous oxide. The degree of hypercapnia and acidosis during sevoflurane anesthesia was similar to that observed during isoflurane anesthesia and less than during halothane anesthesia. The three anesthetic regimens, with or without nitrous oxide, induced a similar degree of hyperglycemia and hemodilution during anesthesia. Serum biochemical examination did not reveal any hepatic or renal injuries after each anesthesia.     

Recovery times and evaluation of clinical hemodynamic parameters of sevoflurane, isoflurane and halothane anaesthesia in mongrel dogs

In the present study the influence of three volatile agents (halothane, isoflurane and sevoflurane) in oxygen at two concentrations [1.5 and 2 minimum alveolar concentration (MAC)] on non-invasive cardio-respiratory parameters (heart and respirators rates, non-invasive blood pressures at 15, 30, 60 min and after extubation) and on the recovery times (appearance of the first eyelid reflex, emergence time) after clinical anaesthesia was studied. After premedication with fentanyl-droperidol (5 microg/kg and 0.25 mg/kg, intramuscularly) and induction with propofol (5 mg/kg, intravenously) six dogs were randomly anaesthetized for 1 h for a standard neurologic stimulation test. A wide individual variation in respiration rate (induced by an initial hyperpnea) was observed in the 1.5 MAC protocols, without significant differences. Heart rate was significantly lower during 1.5 and 2 MAC halothane when compared to isoflurane and sevoflurane. An increase from 1.5 to 2 MAC induced significant decreases in diastolic (DAP) and mean arterial blood pressure in all groups without significant changes in the systolic arterial pressures. Only DAP in sevoflurane protocol was significantly different at 1.5 and 2 MAC compared to halothane. Time had no significant influences in the non-invasive blood pressures in all protocols. Extubation induced a significant increase of all parameters in all protocols. The time for a first eyelid reflex was significantly longer after 2 MAC compared to the 1.5 MAC protocol. There was no significant difference between the three anaesthetic agents. Although emergence time was longest for halothane at both anaesthetic concentrations, no significant difference in emergence time was observed for the three volatile agents.     

Effects of Halothane, Enflurane, and Isoflurane on Supraventricular and Ventricular Rate in Dogs With Complete Atrioventricular Block

Complete atrioventricular (AV) block was produced in 32 chloralose-anesthetized autonomically intact dogs to determine the effects of halothane, enflurane, and isoflurane on supraventricular and ventricular rate. Halothane (n = 17), enflurane (n = 6), and isoflurane (n = 9) were administered in three separate experiments in sequential minimum alveolar concentration (MAC) multiples of 0.5, 1.0, 1.5, 2.0, 1.5, and 1.0. Supraventricular rate, ventricular rate, and mean arterial blood pressure (MAP) were measured and recorded at baseline and after a 20-minute equilibration period of each inhalation anesthetic at each MAC multiple. Increasing concentrations of enflurane and isoflurane significantly decreased supraventricular rate ( P < .05). Ventricular rate was not significantly changed by sequential MAC multiples of halothane, enflurane, and isoflurane. Increasing concentrations of halothane, enflurane, and isoflurane significantly decreased MAP with enflurane producing the most significant decrease ( P < .05). Ventricular arrhythmias occurred in 5 of 17 dogs anesthetized with halothane and 1 of 9 dogs anesthetized with isoflurane. Inhalation anesthesia can significantly decrease supraventricular rate and MAP, does not alter ventricular rate, and can produce ventricular arrhythmias in dogs with complete AV block.     

新型吸入麻醉剂－七氟醚的研究进展II. 兽医药效学

新型吸入麻醉剂七氟醚在犬、猫、马、鼠的MAC分别为2.10～2.36, 2.58, 2.31, 2.40～2.50 Vol％。像异氟醚那样，七氟醚可降低脑血管阻力、脑代谢率、脑耗氧量、心肌收缩功能和血压。七氟醚可引起猫惊厥。虽然猪的心跳在七氟醚麻醉下维持稳定，但研究表明犬的心率则升高。七氟醚麻醉下引起心律失常的肾上腺素剂量与异氟醚相似，但大大高于氟烷或安氟醚。七氟醚对呼吸道的刺激性明显低于其他吸入麻醉剂。尚未见其肝肾毒性。七氟醚已向理想的吸入麻醉剂方向迈出了更进一步。     

Minimal alveolar concentrations for halothane, enflurane, and isoflurane in the cat

The minimal alveolar concentrations for halothane, enflurane, and isoflurane in the domestic cat were found to be 1.19 +/- 0.05 (SEM)%, 2.37 +/- 0.06%, and 1.61 +/- 0.04%, respectively. During the potency studies, it was observed that enflurane and isoflurane resulted in shorter wake-up times, compared with halothane. However, enflurane and isoflurane produced electroencephalographic (EEG) and clinical signs of CNS irritability (EEG spiking, myoclonus) in normocapnic or mildly hypocapnic cats. In addition, enflurane and isoflurane caused greater airway irritability (coughing, salivation) than did halothane.     

新型吸入麻醉剂-七氟醚的研究进展II.兽医药效学

新型吸入麻醉剂七氟醚在犬、猫、马、鼠的 MAC分别为 2 .1 0～ 2 .36 ,2 .5 8,2 .31 ,2 .40～ 2 .5 0 Vol%。像异氟醚那样 ,七氟醚可降低脑血管阻力、脑代谢率、脑耗氧量、心肌收缩功能和血压。七氟醚可引起猫惊厥。虽然猪的心跳在七氟醚麻醉下维持稳定 ,但研究表明犬的心率则升高。七氟醚麻醉下引起心律失常的肾上腺素剂量与异氟醚相似 ,但大大高于氟烷或安氟醚。七氟醚对呼吸道的刺激性明显低于其他吸入麻醉剂。尚未见其肝肾毒性。七氟醚已向理想的吸入麻醉剂方向迈出了更进一步     

Tissue solubility of four volatile anesthetics in fresh and frozen tissue specimens from swine

OBJECTIVE: To determine tissue solubilities of desflurane, sevoflurane, enflurane, and halothane in swine and to evaluate the effects of freezing specimens on tissue solubility, SAMPLE POPULATION: Arterial blood samples and specimens of brain, heart, liver, kidney, muscle, and subcutaneous fat from 5 healthy female adult Chinese Meishan pigs. PROCEDURE: Each tissue specimen was divided into 2 parts. One part was used to measure tissue-gas partition coefficients immediately after collection. The other part was frozen at -20 C for 6 days prior to determination of tissue-gas partition coefficients. Tissue-gas and blood-gas partition coefficients were measured by use of gas chromatography, and tissue-blood partition coefficients were calculated. Regression analysis was performed to determine whether fat-gas partition coefficients were correlated with lean tissue-gas partition coefficients. RESULTS: Tissue-gas and blood-gas partition coefficients of halothane were greater than those of enflurane followed by coefficients of sevoflurane and desflurane. However, the order of anesthetic agents with the greatest to smallest tissue-blood partition coefficients was sevoflurane, halothane, enflurane, and desflurane. Muscle-gas partition coefficients of sevoflurane and enflurane, liver-gas partition coefficients of desflurane and halothane, and the kidney-gas partition coefficient of enflurane were significantly greater in frozen specimens, compared with fresh specimens. Lean tissue-gas partition coefficients of all 4 volatile anesthetics correlated directly with fat-gas partition coefficients. CONCLUSIONS AND CLINICAL RELEVANCE: The fat content of lean tissue is an important factor in determining the tissue solubility of volatile anesthetics. Freezing specimens before determination of tissue-gas partition coefficients may result in a false increase in tissue solubility.     

Degrees of aversion shown by rats and mice to different concentrations of inhalational anaesthetics

The distress associated with the induction of anaesthesia with halothane, isoflurane, enflurane and carbon dioxide was investigated in rats and mice by measuring the level of aversion they displayed on exposure to low, medium and high concentrations of these agents. The animals were exposed to each agent in a test chamber containing air or gas mixtures, which they were able to enter and leave at will, and the level of aversion was assessed in terms of the initial withdrawal and total dwelling times in the chamber. Comparisons between the anaesthetic and air-control treatments indicated that concentrations of the agents recommended for the rapid and efficient induction of anaesthesia were associated with some degree of aversion. Carbon dioxide was by far the most aversive gas for both rats and mice, with the least aversive being halothane for rats, and halothane and enflurane for mice. With all the anaesthetics, the level of aversion increased as the concentration increased.     

Low flow anaesthesia with isoflurane and sevoflurane in the dog

The aim of the present study was to compare the safety and efficacy of sevoflurane and isoflurane during low flow anaesthesia (fresh gas flow (FGF) 14 ml/kg/min) as well as to compare the consumption of both anaesthetics. Data were gathered from 60 dogs assigned for surgery under general anaesthesia with an expected duration of 75 minutes or longer. All dogs were induced with 0.6 mg/kg (maximum 25 mg) l-methadone and 1 mg/kg (maximum 25 mg) diazepam i.v.. Anaesthesia was maintained with isoflurane (group 1) or sevoflurane (group 2) in a mixture with 50% O2 and 50% N2O as carrier gases, under controlled ventilation. Monitoring included electrocardiogram, body temperature, the temperature of in- and exspired gases, arterial oxygen saturation, arterial blood pressure as well as a continuous monitoring of inhaled and exhaled gas concentrations (O2, N2O, CO2, isoflurane, sevoflurane). The consumption of isoflurane and sevoflurane as well as the dogs' recovery times were evaluated for both groups. In all groups the inspired oxygen concentrations ranged above the minimum value of 30 Vol% during low flow anaesthesia, with an arterial oxygen saturation above 97%. End tidal concentration of CO2, heart rate and arterial blood pressure were within the physiological ranges and showed no differences between the two groups. Recovery time was significantly shorter after sevoflurane compared to isoflurane anaesthesia, whilst the consumption of sevoflurane was higher than that of isoflurane. Sevoflurane appears to be as clinically safe as isoflurane in low flow anaesthesia. Even considering that sevoflurane is more expensive than isoflurane, the use of the low flow technique decreases the cost of anaesthesia due to the reduced volatile anaesthetic consumption.     

Influence of halothane, isoflurane, and sevoflurane on gastroesophageal reflux during anesthesia in dogs

OBJECTIVE: To determine whether maintenance of anesthesia with halothane or sevoflurane is associated with a lower incidence of gastroesophageal reflux (GER) than the use of isoflurane in dogs undergoing orthopedic surgery. ANIMALS: 90 dogs. PROCEDURES: Dogs were evaluated during elective orthopedic surgery. Dogs with a history of vomiting or that had received any drugs that would alter gastrointestinal tract function were excluded from the study. The anesthetic protocol used was standardized to include administration of acepromazine maleate and morphine prior to induction of anesthesia with thiopental. Dogs were allocated to receive halothane, isoflurane, or sevoflurane to maintain anesthesia. A sensor-tipped catheter was placed to measure esophageal pH during anesthesia. Gastroesophageal reflux was defined as an esophageal pH < 4 or > 7.5. RESULTS: 51 dogs had 1 or more episodes of acidic GER during anesthesia. Reflux was detected in 14 dogs receiving isoflurane, 19 dogs receiving halothane, and 18 dogs receiving sevoflurane. In dogs with GER, mean +/- SD time from probe placement to onset of GER was 36 +/- 65 minutes and esophageal pH remained < 4 for a mean of 64% of the measurement period. There was no significant association between GER and start of surgery or moving a dog on or off the surgery table. Dogs that developed GER soon after induction of anesthesia were more likely to regurgitate. CONCLUSIONS AND CLINICAL RELEVANCE: Maintenance of anesthesia with any of the 3 commonly used inhalant agents is associated with a similar risk for development of GER in dogs.     

Effects of isoflurane,sevoflurane and methoxyflurane on the electroencephalogram of the chicken

ObjectiveAnaesthetics have differing effects on mammalian electroencephalogram (EEG) but little is known about the effects on avian EEG. This study explored how inhalant anaesthetics affect chicken EEG.Study designExperimental study.AnimalsTwelve female Hyline Brown chickens aged 6–11 weeks.MethodsEach chicken was anaesthetized with isoflurane, sevoflurane, and methoxyflurane. For each, anaesthesia was adjusted to 1, 1.5 and 2 times Minimum Anaesthetic Concentration (MAC). Total Power (Ptot), Median Frequency (F50), Spectral Edge Frequency (F95) and Burst Suppression Ratio (BSR) were calculated at each volume concentration. BSR data were analyzed using doubly repeated measures anova. Neither isoflurane nor sevoflurane could be included in analysis of F50, F95 and Ptot because of extensive burst suppression; Methoxyflurane data were analyzed using RM anova.ResultsThere was a significant interaction between anaesthetic and concentration on BSR [F(4,22) = 10.65, p < 0.0001]. For both isoflurane and sevoflurane, BSR increased with concentration. Isoflurane caused less suppression than sevoflurane at 1.5 MAC and at final 1 MAC while methoxyflurane caused virtually no burst suppression. Methoxyflurane concentration had a significant effect on F50 [F(2,20) = 3.83, p = 0.04], F95 [F(2,20) = 4.03, p = 0.03] and Ptot [F(2,20) = 5.22, p = 0.02]. Decreasing methoxyflurane from 2 to 1 MAC increased F50 and F95. Ptot increased when concentration decreased from 1.5 to 1 MAC and tended to be higher at 1 MAC than at 2 MAC.Conclusions and Clinical relevanceIsoflurane and sevoflurane suppressed chicken EEG in a dose-dependent manner. Higher concentrations of methoxyflurane caused an increasing degree of synchronization of EEG. Isoflurane and sevoflurane suppressed EEG activity to a greater extent than did methoxyflurane at equivalent MAC multiples. Isoflurane caused less suppression than sevoflurane at intermediate concentrations. These results indicate the similarity between avian and mammalian EEG responses to inhalant anaesthetics and reinforce the difference between MAC and anaesthetic effects on brain activity in birds.     

Comparison of T-cell subpopulations in cats naturally infected with feline leukaemia virus or feline immunodeficiency virus

T-cell subsets were studied by flow cytometry in 58 feline leukaemia virus (FeLV)-positive cats with naturally acquired FeLV infection to determine whether the changes in CD4⁺ or CD8⁺ T cell populations differed from those observed in 55 feline immunodeficiency virus (FIV)-positive cats with naturally acquired FIV infection. The sole criterion for inclusion into the study was seropositivity. Mean (SD) CD4⁺ T cell values of FeLV positive cats were decreased to 31·1 (8·0) per cent and their CD8⁺ T cell values were increased to 22·8 (6·3) per cent in comparison with uninfected control cats (37·9 [9·5] per cent CD4⁺; 15·2 [6·3] per cent CD8⁺). The CD4⁺/CD8⁺ ratio was reduced to 1·5 (0·7), compared with 3·0 (1·5) in 39 FeLv- and FIV-negative control cats. Differences from control values were significant, but there was no significant difference between CD4⁺ and CD8⁺ lymphocytes of FeLV- versus FIV-infected cats. These findings indicate that FeLv and FIV have similar effects on T lymphocyte subsets. Both retrovirus infections can induce immunodeficiency, both viruses infect a broad range of lymphohaemopoietic cells, despite having different primary target cells, and can induce the killing of lymphocytic cells in vitro. It is concluded that a decreased CD4⁺/CD8⁺ ratio is not restricted to FIV infections but may also occur in FeLv infection.     

Effect of halothane, isoflurane, and pentobarbital anesthesia on myocardial irritability in chickens

The relative myocardial irritant properties of halothane, isoflurane, and pentobarbital were evaluated in chickens. Sixteen adult male broiler chickens were randomly assigned to 1 of 3 groups: group-1 chickens were anesthetized with pentobarbital (30 mg/kg, IV), group-2 chickens were anesthetized with halothane (end tidal halothane 1.2%), and group-3 chickens were anesthetized with isoflurane (end tidal isoflurane 2.1%). Birds in any 2 of the 3 treatment groups were tested on any 1 day. Local anesthesia was induced, and blood pressure, heart rate, ECG, and blood gas variables were measured before general anesthesia was induced. Positive-pressure ventilation with an inspired O2 fraction greater than 0.95 was adjusted to result in an end tidal CO2 concentration that reflected a Paco2 similar to that obtained prior to anesthesia and ventilation. All measurements were repeated. The threshold for ventricular fibrillation in response to electrical stimulation of the heart was then determined for all birds. Effects of anesthesia on hemodynamic and blood gas variables were similar in all 3 groups. Compared with halothane or pentobarbital, isoflurane anesthesia resulted in a significantly (P less than 0.05) lower threshold for electrical fibrillation of the heart.     

Effects of halothane and isoflurane on mean arterial blood pressure, heart rate, and respiratory rate in adult Pekin ducks

Although the cardiovascular and respiratory effects of halothane and isoflurane have been documented in a variety of common mammalian laboratory animals, they have not been investigated in birds. In this study, the effects of halothane and isoflurane anesthesia on respiratory rate, heart rate, heart rhythm, and mean arterial pressure in adult Pekin ducks were evaluated. Both anesthetics significantly increased heart rate and depressed blood pressure and respiration. Halothane induced a more profound alteration in heart rate and respiratory rate. With the ducks under halothane anesthesia, abnormal cardiac rhythms included ventricular fibrillation, ventricular bigeminy, and multifocal ventricular rhythms. Other than cardiac tachycardia, isoflurane induced no changes in cardiac rhythm.     

Clinical comparison of isoflurane and sevoflurane anaesthesia in the gerbil (Meriones unguiculatus)

The objective of this study was a comparison of the volatile anaesthetics isoflurane and sevoflurane in terms of their clinical effects in gerbils (Meriones unguiculatus) (n=12 each). Induction of anaesthesia was performed in a body chamber with an anaesthetic concentration of 4.0 Vol.% at an oxygen flow of 500 ml/min for isoflurane and 8.0 Vol.% at an oxygen flow of 1000 ml/min for sevoflurane, respectively. Anaesthesia was maintained via nose cone with an anaesthetic concentration of 2.8 to 3.2 Vol.% at an oxygen flow of 200 ml/min for isoflurane and 5.0 to 5.2 Vol.% at an oxygen flow of 400 ml/min for sevoflurane. Those anaesthetic concentrations ensured reflex status conform with surgical tolerance. In spite of its higher blood-gas coefficient induction time was slightly faster for isoflurane. Recovery time was significantly longer in the isoflurane group than it was in the sevoflurane group. Both inhalants caused respiratory depression. Respiratory rate was lower in sevoflurane animals compared to isoflurane. The animals were positioned on a heating pad immediately after induction, thus a decrease of the body temperature could be prevented. Both inhalants can be recommended for usage in gerbils. Sevoflurane showed no clinical benefit compared to isoflurane.     

Effects of volatile anesthetics on the activity of laryngeal 'drive' receptors in anesthetized dogs.

Effects of halothane, isoflurane and sevoflurane on laryngeal drive receptor activity were studied in the afferent activity of the superior laryngeal nerve in anesthetized spontaneously breathing dogs. Of 40 single units recorded, most of them (65%) responded to the volatile anesthetics applied to the isolated larynx at a concentration of 5%. The exposure to the anesthetics resulted in either an inspiratory increase (15%), both inspiratory and expiratory decrease (54%), or both inspiratory increase and expiratory decrease (31%) responses. The average discharge frequency of the receptors tended to be decreased on inhalation of the anesthetics, where significant decreases were observed in both respiratory phases for halothane and at expiration for isoflurane, but in neither respiratory phase for sevoflurane. These results support an advantage of sevoflurane over halothane and isoflurane for induction of anesthesia to minimize the influence of the activity of laryngeal drive receptors on the breathing pattern and airway stability.     

Effect of inhalation anaesthetics on total respiratory resistance in conscious ponies

Total respiratory resistance was measured rapidly and non-invasively in 6 conscious ponies before and after they inhaled approximately 25% of the minimal anaesthetic concentration (0.25 MAC) of either enflurane, halothane, or isoflurane, over a 10 min period. The forced random noise (FRN) method was used to measure the impedance over the frequency range of 5 to 40 Hz and its real part, the resistance, was extracted from these impedance measurements. At the concentrations used, halothane appeared to have no effect on the total respiratory resistance; enflurane and isoflurane seemed to increase it but the changes were not statistically significant.